Glass, for many years, has been manufactured by a process wherein particulate, glass forming batch ingredients were dispensed into a glass melter, for example a fossil fuel fired melter, then vitrified and formed into articles of commerce. More recently the glass industry, like society as a whole, has become acutely aware of the shortages of energy and the need to improve the quality of our environment or atmosphere. Consequently, steps have been taken to provide for glass manufacturing processes which consume lesser amounts of energy, minimize pollution, and increase the throughput, or output, of the melting furnaces employed. One technique which has been developed to accomplish this involves combining glass batch ingredients and water into agglomerates, for example, pellets or briquettes, and then conveying hot effluent gases from above a pool of molten glass in a melter through a bed of such agglomerates so as to dry and preheat the agglomerates, and thereby recover otherwise wasted energy, and likewise to recover, in the bed of agglomerates, materials which otherwise could pollute the atmosphere. The individual preheated agglomerates are then fed to the melting furnace and melted. While such processes have improved the efficiency of melting, for example, by providing for higher throughputs per square foot of melter area, and have decreased the amount of wasted energy, such processes still need improvement in their pollution abating characteristics.
With the foregoing in mind, it is the general object of the present invention to provide for a glass manufacturing process which makes efficient use of energy, has a high melter throughput, and which has improved pollution abating characteristics. More specifically, it is the object of this invention to improve the recovery of boron values and/or fluorine values carried by furnace effluent gases to thereby minimize atmospheric pollution. These values are then recycled into the melter and, consequently, once the process reaches equilibrium, an adjustment may be made to the batch composition to employ lesser amounts of glass making raw materials.
Thus, in accordance with the present invention, there is provided an improvement in glass manufacturing processes of the type comprising combining glass batch ingredients, including a source of fluorine and/or boron, with water into agglomerates, conveying hot, boron and/or fluorine containing, effluent gases from above a pool of molten glass to a bed of agglomerates and passing such gases directly therethrough so as to preheat the agglomerates to an elevated temperature, for example a temperature in excess of at least about 500.degree. C., feeding the preheated agglomerates to a glass melting furnace and melting the agglomerates therein. The improvement essentially resides in introducing a fluorine and boron reactive material into the hot gases prior to passing the gases through the bed of agglomerates, reacting boron and fluorine values in the gases with said material at a temperature, for example, in excess of about 500.degree. C. and then recovering the reaction product thereof in the bed upon passage of the gases therethrough. As used herein, the terms "boron" and "fluorine" generally comprehend any compounds existing in the glass manufacturing process which contain boron or fluorine and includes even the elemental forms. Preferred sources of boron and fluorine for a glass manufacturing batch as provided herein will be colemanite which has been treated to remove substantially all of its chemically bound water, i.e., calcined colemanite, and sodiumsilicofluoride. Exemplary of the forms in which boron and fluorine may exist when being conveyed in the flue gases from the melting furnace are the compounds H.sub.3 BO.sub.3, HBO.sub.2, HF, and BF.sub.3.
In accordance with another feature of this invention, there is provided an improvement in glass manufacturing processes of the type comprising providing a gravitationally downwardly flowing vertical bed of glass batch agglomerates, said agglomerates containing a source of fluorine and/or boron, passing flue gases emanating from a combustion fired melting furnace directly through said vertical bed to preheat the agglomerates, said flue gases containing fluorine and/or boron values, and melting said preheated agglomerates in the combustion fired furnace. The improvement comprises decreasing the fluorine and/or boron content of the gases after passage through the bed by reacting the fluorine and/or boron values in the gases, prior to passage through said bed and while the gases generally have an absolute humidity substantially no higher than their absolute humidity upon leaving the furnace, with an alkaline earth metal oxide, or a precursor compound of such oxide. In so reacting the boron and/or fluorine, the reaction products are then recovered in the agglomerate bed and recycled into the melting furnace. In this way, the amount of boron in the flue gases after passage through the preheating bed can be decreased by as much as 30 or 35 percent or even more, and the amount of fluorine can be decreased by as much as 15 or 20 percent or even more.